Nanowire structural element

ABSTRACT

The invention concerns a nanowire structural element suited for use in a microreactor system or microcatalyzer system. A template based process is used for the production of the nanowire structural element, wherein the nanowires are electrochemically depositioned in the nanopores. The irradiation is carried out at different angles, such that a nanowire network is formed. The hollow chamber-like structure in the nanowire network is established through the dissolving of the template foil and removal of the dissolved template material. The interconnecting of the nanowires provides stability to the nanowire structural element and an electrical connection between the nanowires is created thereby.

FIELD OF THE INVENTION

The invention concerns a nanowire structural element, a process forproduction of said and a micro-reactor system, specifically amicrocatalyzer system.

BACKGROUND OF THE INVENTION

In “Chemistry in Microstructured Reactors,” Ang. Chem. Int. Ed. 2004,43, 406-466 [: Applied Chemistry, International Edition], K. Jähnisch etal. have demonstrated the advantages that microstructured componentshave in chemical reactions and for analytical purposes. This has led toan increase in the importance that such systems have for chemicalsynthesis and analysis. In comparison to conventional reactors, thesemicrostructures have a large surface area/volume ratio, which has apositive influence on the transference of heat as well as the process ofthe transportation of matter (see also: O. Wörz et al. “Micro-reactors—ANew Efficient Tool for Reactor Development,” Chem. Eng. Technol. 2001,24, 138-142).

Many known reactions have been carried out in microstructure reactors,including many catalytic reactions. For these, it is unimportant whetherthe reactions are liquid phase, gas phase or gas-liquid phase reactions.In order to take advantage of the potential activity of the catalyzer,the catalytic material is integrated in microstructured systems withvarious geometric forms. In the simplest case, the reaction materialused for the construction of the micro-reactor consists in itself of thecatalytically active substance (see also: M. Ficthner, “MicrostructuredRhodium Catalysts for the Partial Oxidation of Methane to Syngas underPressure,” Ind. Eng. Chem. Res. 2001, 40, 3475-3483). This means howeverthat the catalytic surface is limited to the walls of the reactor. Thisdisadvantage is partially resolved by means of optimizedcatalyzer/carrier systems. For the most part, current micro-structurereactors contain small particles or powder, which are incorporated in achannel.

Catalyzer filaments, wires and membranes are also used however (seealso: G. Veser, “Experimental and Theoretical Investigation of H₂Oxidation in a High-Temperature Catalytic Microreactor,” Chem. Eng. Sci.2001, 56, 1265-1273). Metallic nanostructures, particularly those fromtransition metals, are known in heterogenic catalysis due to their highratio of surface area/mass, resulting in lower production costs (seealso: R. Narayanan et al. “Catalysis with Transition Metal Nanoparticlesin Colloidal Solution: Nanoparticle Shape Dependence and Stability,” J.Chem. Phys. B, 2005, 109, 12633-12676).

Originally, research was concentrated on the examination of isotropicmetal particles, and as a result, their catalytic characteristics havebeen studied at length. At present, however, many one-dimensionalnanostructures have been analyzed regarding their use in heterogeniccatalysis. The stabilization of these is a major problem. Theincorporation of nanostructures on a carrier or storage of them inporous matter such as, e.g. Nafion is known from Z. Chen et al.“Supportless Pt and PtPd Nanotubes as Electrocatalysts forOxygen-Reduction Reactions,” Ang. Chem. 2007, 119, p. 4138-4141, whichleads however directly to a decrease in the utilizable catalyzer surfacearea. Furthermore, it must be noted that the catalytic activity isdependent on the distribution of the catalyzer material due to thediffusion processes. Accordingly, the nanoparticles significantlyincrease the surface area/volume ratio, but long-term stability of suchreactors is relatively limited due to the following:

1. Loss of contact between nanoparticles due to corrosion of thecarrier.

2. Dissolving and renewed deposition or Ostwald ripening.

3. Aggregation of the nanoparticles in order to minimize the surfaceenergy.

4. Dissolving of the nanoparticles and migration of the dissolvableions.

Parallel wire and tube structures have already been used as glucosesensors (J. H. Yuan et al., “Highly Ordered Platinum-Nanotubule Arraysfor Amperometric Glucose Sensing,” Adv. Funct. Mater. 2005, 15, 803), aselectrocatalysts, for example, in alcohol oxidation (H. Wang et al., “PdNanowire Arrays as Electrocatalysts for Ethanol Electrooxidation,”Electrochem. Commun. 2007, 9, 1212-1216) and for hydrogen peroxidereduction (H. M. Zhang et al., “Novel Electrocatalytic Activity inLayered Ni—Cu Nanowire Arrays,” Chem. Cornimm. 2003, 3022). In thesecases however, the nanostructures are not particularly stable.

Nielsch et al. have reported in “Uniform Nickel Deposition into OrderedAlumina Pores by Pulsed Electrodeposition,” Adv. Mater. 2000, 12,582-586, that pulsed deposition is used for deposition of thin metallicfoils.

A process for the generation of nanowires known from, for example, T. W.Cornelius et al., “Controlled Fabrication of Poly- andSingle-Crystalline Bismuth Nanowires,” Nanotechnology 2005, 16, p.246-249; or from the dissertations by Thomas Walter Cornelius, GSI,2006; Florian Maurer, GSI, 2007 and Shafgat Karim, GSI, 2007, which arehereby incorporated as references. With these processes however, onlysingle nanowires were obtained.

GENERAL DESCRIPTION OF THE INVENTION

The invention has the object of providing a complex nanowire structuralelement and a process for the production of said which has a stablehollow chamber-like structure with a large specific surface area.

A further object of the invention is to provide a nanowire structuralelement of this type which may be used in a number of ways, e.g. as acatalytic element.

The object of the invention is achieved by means of the object of theindependent claims. Advantageous embodiments of the invention aredefined in the dependent claims.

A process is provided for the production of a nanowire structuralelement which contains a nanowire array of numerous nanowires, whereinthe nanowires run in different directions and the nanowires running indifferent directions intersect, thereby forming a meshed network ofnanowires. The meshed network forms, thereby, an open cell hollowchamber-like structure with a very large interaction surface area. Thehollow chamber-like structure may be envisioned as a chamber that can beopen at one or more edges.

For production a so-called template based process is used as follows.

In a first process step (a), first, a dielectric template, in particulara dielectric template foil, is prepared. The template foil is, forexample, a conventional commercially available synthetic foil, inparticular, a polymer foil.

In a subsequent process step (b) the template foil is irradiated (c1)with high-energy radiation, in particular with a highly energetic ionradiation, such as is available, for example, in the acceleratorfacility of the Gesellschaft far Schwerionenforschtmg mbH [: Center forHeavy Ion Research; abbreviation: GSI] in Darmstadt. As a result of theirradiation a large number of latent tracks permeate the template foil.The tracks are characterized in that the molecular structure, e.g. thepolymer structure of the foil is corrupted along the trajectory of eachirradiation ion. These tracks are referred to as “latent tracks.” Thedamage is greatest at the core of the track and is 1/r². Using etchingtechniques, the material having a corrupted molecular structure can beremoved from the track and the latent track becomes by this means anopen channel, or respectively, a so-called nanopore can be etched. Thelatent tracks and thereby the subsequently generated nanopores arestochastically distributed in relation to the plane of the templatesurface.

In accordance with the invention, the template, or respectively, thetemplate foil in step (b) is irradiated from at least two, preferablythree or more, angles to the surface of the template—in other words,from at least two, preferably three or more different directions. Inthis manner latent tracks are first created in the template foil thatrun in at least two, preferably three or more different, non-paralleldirections. For example, a polycarbonate foil is irradiated with heavyions having an energy ranging from a few to a few tens MeV/u at twoangles (+45°, −45°) in relation to the surface plane of the templatefoil.

Particularly preferable thereby is that the template foil is irradiatedfrom at least three (or more) different directions which are not in thesame plane, wherein a three-dimensional interconnected nanopore networkcan be produced, which will be explained in greater detail in thefollowing. For example, a polycarbonate foil is irradiated with heavyions having an energy of a few to a few hundred MeV/u from threedifferent directions, e.g. at a polar angle in each case of 45° to thesurface plane and with an azimuth angle at 0°, 120° and 240° to thesurface plane with a fluence of a few 10⁸ ions/cm² in each case. It isclear that the complexity of the network can be increased through moreirradiation angles. In this case, the energy of the ions is selectedsuch that said fully penetrate the foil. The energy of the ion beam isthereby dependent on the thickness of the foil which is to beirradiated.

Preferably, an electroconductive metallic layer as, if applicable, atemporary, cathode layer is applied to the first side of the templatefoil and this, preferably after the ion irradiation has been carriedout, and further preferably before etching, but at least prior to theelectrochemical deposition. Preferably, therefore, the nanopores areetched from the latent ion induced tracks after the cathode layer is atleast partially applied to the template foil. In this manner, materialfrom the cathode layer being depositioned in the pores is avoided.Furthermore, the pores are particularly strictly cylindrical and do nottaper at either end.

Preferably, for the production of the cathode layer, first a thin metallayer is applied to the first side of the template foil, e.g. a goldlayer is sputtered onto said, and subsequently this gold layer isreinforced with, for example, a copper layer. This has the advantagethat first a relatively thin layer can be sputtered.

In the process step (c) the template foil is subjected to an etchingprocedure wherein the latent tracks in the template foil are enlarged toform continuous channels which fully penetrate the template foil,thereby reaching from one surface of the template foil to the oppositesurface of the template foil. These channels are referred to in thefield as nanopores. At the beginning of the etching procedure the mostcorrupted part of the molecular structure at the track core is dissolvedaway and with increasing etching periods the diameter of the nanoporesis increased. Due to the irradiation of the template foil from differentdirections and the resulting intersecting network of latent tracks, anetwork of intersecting nanopores is formed through etching. Theintersecting nanopores are so densely arranged that a significantportion of the intersecting nanopores are interconnected such that aninterconnected channel system with numerous branches is formed. Whenirradiation is carried out from at least three directions not lying in acommon plane, a three-dimensional interconnected channel system ofnanopores is formed.

In a subsequent step (d), starting at the inner side of the cathodelayer, nanowires are grown in the nanopores within the template foil bymeans of electrochemical deposition, i.e. the nanopores are filled bythe cathode layer by means of electrochemical deposition, wherein thenanowires grow in the nanopores. For this purpose, the dielectric foil,permeated with pores and electroconductively coated on one side, isplaced in an electrochemical deposition device, wherein the cathodelayer serves as a cathode for the electrochemical deposition procedureof the nanowires. The nanowires are grown in the nanopores from metalions by means of electrochemical deposition, wherein the metallicnanowires develop inside the nanopores, in particularly, directly on thecathode layer and are thereby integrally joined to the cathode layer byreason of being grown together. The cathode layer may remain as asubstrate layer firmly joined to the individual nanowires of thenanowire structural element to be generated, but may also, if desired,be removed after the generation of the nanowire network. In the meshednetwork of nanopores or nanochannels, a meshed network of intersectingnanowires is formed thereby. At the nodes at which the nanopores areinterconnected, nodes in the nanowire network are formed accordingly, atwhich the intersecting metal nanowires grow together, or respectively,become integrally joined with each other. In this manner an integratednetwork of intersecting nanowires joined together can be generated. Thetemplate foil at this point of the process is permeated by the connectednetwork of intersecting and interconnected nanowires in the manner ofreinforced concrete.

The nanowires develop therefore inside the nanopores in the templatefoil in at least two, at least three, or more, predefined differentdirections which are determined by the direction of the irradiation. Ifthe irradiation is carried out from at least three directions which donot lie in a common plane, then the nanowires in the network run alongat least three predefined different directions which are not in a commonplane, thus producing a three-dimensional interconnected nanowirenetwork. In a three-dimensional interconnected nanowire network, thenanowires of a first of the, at least three, predefined directionsaccordingly are connected to other nanowires of not only a secondpredefined direction but also with nanowires of at least one thirddirection, wherein the third direction does not lie in the planesthrough which the first and second directions run.

When the network of nanowires is completely depositioned, orrespectively, matured, the template foil is dissolved, in particularlyby chemical means, in a step (e) and the network of nanowires is therebyexposed. The template foil is reduced in the dissolving to such smallcomponents that these components can be removed from the space permeatedby the nanowire network without damaging the nanowire network. If thetemplate foil is a synthetic foil, this can, for example, be dissolvedwith a solvent. Due to the meshing of the network, or respectively, theintegral joining of the intersecting nanowires, the integrally joinednanowire network has an inherent stability even after the removal of thetemplate foil, and this with only one cover layer (cathode/substratelayer) and, if applicable, even without a cover layer. The joinednetwork of nanowires forms an open cell hollow chamber-like structurewhich is at least inherently stable, which can be handled, carefully,relatively well.

This means that in accordance with the invention, the cathode layer caneven be fully removed after the network of nanowires is fullydepositioned, or respectively, formed, if desired. The cathode layer isthen, in particular, removed from the template foil after the depositionof the nanowire network and prior to the dissolving and removing of thetemplate foil. This is, for example, particularly possible when thecathode layer and the nanowire network are of different metals wherethey meet. The removal of the cathode layer is however not necessary,and can remain integrally joined to the nanowire network if, forexample, a closed cover layer integrally joined to the nanowire networkis desired. If the cathode layer remains on the nanowire network, itforms a substrate layer which also increases the stability and makes thenanowire structural element easier to handle. Preferably, the nanowirestructural element forms thereby a flat mat-like shape. In particular,it is also possible to produce a flat nanowire structural element, whichon at least one flat side of the nanowire network does not have a coverlayer, and is therefore open and the opposite flat side of the nanowirenetwork is firmly joined to the substrate layer, or respectively,unified with said. In this case, the individual nanowires are in eachcase joined to the substrate layer individually. The flat sides of theflat mat-like nanowire structural element are defined by the surfaces ofthe template foil. As a result of the stable interconnection (mechanicalintegral joining at the nodes) of the nanowires, it is accordingly alsopossible to produce a nanowire network without any cover layers.

In order to obtain a sufficient nanowire density and stability of thenanowire network, the ion irradiation is carried out with an ion beamintensity which is sufficiently high enough that a sufficiently largenumber of intersecting nanopores at the nodes overlap, such that theintersecting nanowires merge together at enough nodes of theinterconnected network. The intensity of the ion beam, or moreprecisely, the surface density (number of ions per surface unit) foreach irradiation direction ideally should be at least 1×10⁷ ions/cm²,preferably, at least 5×10⁷ ions/cm², and particularly preferred is atthe rate of 5×10⁸ ions/cm². The intensity of the ion beam should atleast be high enough that a network is formed in which in the middle atleast one node, in particular even more nodes, for each nanowire is/aregenerated. The nodes of the intersecting nanowires thereby, are betweenthe ends of the nanowires, and due to the stochastic distribution of theions during the irradiation are at different points along the length ofthe nanowires for each nanowire.

One advantage of the invention, among others, is that a nanowire networkcan be generated which is open on at least four sides, or at least fivesides, or even on all sides (like a sponge) and is still stable and forthe most part freestanding. The nanowire network forms thereby, inparticular, a stable, or respectively, freestanding nanowire structuralelement. After complete removal of the template foil, an accordinglystructurally stable hollow chamber-like structural component with ananowire network structure remains.

These nanowire structural elements having a nanowire network open on allsides or merely closed by the substrate layer on one side or nanowirestructural elements closed on two flat sides are excellently suited foruse as, for example microreactor components, in particular asmicrocatalyzer components for heterogeneous catalysis. Furthermore, thenanowire structural element has a high level of long term stability asthe nanowires are firmly anchored at numerous nodes to each other, andare not, for example, lying loosely in a microchannel. Even if theindividual nodes should disconnect, the number of the remaining nodes isstill sufficient to ensure the stability of the network.

The nanowires are preferably pulsed depositioned. The pulsed depositionhas at least the following alternatives:

1) The deposition is carried out using pulsed deposition, i.e.deposition pulses alternating with deposition free diffusion periods.

2) The deposition is carried out by means of reversed pulse deposition,i.e. deposition pulses alternating with anodic counter-pulses.

Both alternatives have the advantage that in the breaks between thedeposition pulses, ions in the electrolyte solution can re-diffuse inthe nanopores, which leads to a uniform development of the nanowires.

The cathode layer can be generated by means of a known coating process,which is suited for application of a conductive, e.g. metallic, layer(e.g. vaporization, PVD, sputtering etc.). The cathode layer can begenerated thereby in a single layer. Ideally however, the cathode layeris generated in at least two layers, wherein the first partial layer isapplied through deposition, e.g. by means of PVD, sputtering, orvaporization and said first partial layer is then reinforced with asecond partial layer by means of electrochemical deposition of anothermaterial, e.g. copper on gold.

The result of the production process described above is, accordingly, ananowire structural element with a hollow chamber-like structurecontaining an array of numerous intersecting or angled nanowires whichare merged together at numerous nodes. Merged, in this case, means thatthe nanowires are integrally joined to each other at an atomic/molecularlevel by means of the electrochemical deposition. The meshed nanowirenetwork is therefore a unified developed system of matter fromelectrochemically depositioned material.

Interconnected open spaces exist between the nanowires in the nanowirenetwork. The hollow chamber-like structure formed by the nanowirenetwork is accordingly open celled, such that a fluid can be fed throughthe open cell hollow chamber-like structure in order to interact withthe cylindrical surfaces of the nanowires which form a large surfacearea.

By means of the production process, there are however further certainstructural properties of the constructed nanowire structural element.Because the nanowires, or respectively, the nanowire network are/isgenerated from electrochemical deposition materials, they can have aspecific crystal structure which, for example, can be examined by meansof X-ray diffraction. In this manner, based on the crystal structure, itis possible to determine whether the nanowire network was produced usingthe described process.

The diameter of the nanowires is preferably less than or equal to 2,000nm, particularly preferably less than or equal to 500 nm, orrespectively less than or equal to 100 nm. It currently seems possibleto produced diameters as small as 10 nm or even less.

A larger aspect ratio allows for the production of a larger activesurface area of the nanowire structural element. The aspect ratio of thenanowires is therefore ideally greater than or equal to 1:50,particularly preferred is greater than or equal to 1:100. It is alsopreferable that the average distance between the nanowires is greaterthan the average diameter of the nanowires.

The thickness of the nanowire structural element is defined by thethickness of the template foil. Due to the flatness of the templatefoil, the nanowire structural element is preferably also flat, or in theform of a thin mat. The thickness of the nanowire network is ideallyless than or equal to 200 μm, particularly preferred is less than orequal to 50 μm. The dimensions in the two planes perpendicular to thethickness plane can amount to many times this. It is, for example,possible to produce a nanowire network of this type with a surface of0.5 cm².

The surface density of the number of nanowires corresponds basically tothe irradiation density (ions/cm²) is equally a measure for the activesurface area. The surface density of the number of nanowires is ideallygreater than or equal to n/F=10⁷ cm⁻², particularly preferred is greaterthan or equal to n/F=10⁸ cm⁻².

As a specific size for the active surface area of the nanowirestructural element, the geometric specific surface of the nanowires perarea of the nanostructure element and per length of the nanowires may beused. Accordingly, this geometrically specific surface area A_(v) is:

$A_{v} = {\pi \cdot \frac{nD}{{F\;\cos} \propto}}$Wherein D is the average diameter of the nanowire and n/F is the surfacedensity of the nanowires and α is the average angle of the nanowire tothe surface plane of the template foil.

The geometrically specific surface area A_(v) for each surface isdetermined by the total number of nanowires and should be at least 1mm²/(cm² μm); larger values however are preferred, specifically whereA_(v) is greater than or equal to 5 mm²/(cm² μm), greater than or equalto 20 mm²/(cm² μm) or even greater than or equal to 100 mm²/(cm² μm).Where applicable, values of up to 1000 mm²/(cm² μm) may even beobtained.

In the production of the nanowires with the reversed pulse process, thenanowires have a distinct <100> texture, or respectively, a crystallinestructure. With certain metals such as, for example, gold, it may beadvantageous to create the smallest crystallite possible. For this acrystallite size of less than or equal 4 nm is preferred, wherein ingeneral an average crystallite size of less than or equal to 10 nm maybe advantageous.

Due to the crystalline texture, the actual size of the surface area,particularly the active surface area, is larger than the geometricallyspecific surface area A_(v), which is based on the smooth cylindricalsurface area, ideally by a factor of around 4-5.

A particularly preferred field of application for the nanowirestructural elements produced according to the invention is heterogeniccatalysis. This means one or more components serve as catalyticcomponents, particularly for microcatalyzers.

A microcatalyzer ideally contains a microstructured channel system witha fluid intake and a fluid discharge and at least one nanowirestructural element as a catalyzer element between the fluid intake andthe fluid discharge, in order that fluid may be introduced by means ofthe fluid intake to the hollow chamber-like structure between the twocover layers, fed through the spaces between the nanowires and thenremoved by means of the discharge from the hollow chamber-likestructure. In this manner, the two-dimensional open cell hollowchamber-like structure of the nanowire structural element forms thecatalytic reaction volumes and the cylindrical surfaces of the nanowireform the catalytically active surface area which interacts with thefluid within the hollow chamber-like structure. Ideally, due todeposition, the nanowires are formed significantly (entirely of the samematerial) of, for example, platinum, in order that the catalytic elementis a fully catalytic element.

Even when, due to the particular stability of the nanowire network, itis not actually necessary to apply cover layers to both sides of thenanowire structural element, this possibility should not be eliminatedif it is desired, as, for example, in the following: the electrochemicaldeposition procedure of the nanowires is carried out at least until capshave formed on the second side of the template foil, and said caps mergeto form a second closed cover layer on the side opposite the cathodelayer. This surface covering cover layer increases in thickness incorrelation to the length of time of the deposition. Accordingly, theelectrochemical deposition procedure used to form the nanowires can becarried out long enough that the second cover layer in the form of asufficiently thick, stable surface covering layer has formed. Thenanowires are firmly joined in this case at both ends with therespective cover layers as a result of the electrochemical deposition.The cover layers may be, if desired, reinforced and completed as well,however, in a separate second, subsequent deposition procedure. Theseparate second deposition procedure can also be an electrochemicaldeposition, but can also consist of a coating process, such as PVDprocesses, vaporization or sputtering. Even when the separate depositionprocedure is an electrochemical deposition, a different material may beused for the second partial layer than that used for the nanowires andthe caps. It has been shown to be particularly beneficial if thenanowires and the caps are generated using a pulsed electrochemicaldeposition and the second partial layer is depositionedelectrochemically using a direct current process. In particular, thenanowires and where applicable, the caps, are generated, using areversed pulse deposition, from a metallic compound. For example, it hasbeen shown to be beneficial to produce the nanowires and, if applicable,the caps, using reversed pulse deposition, from platinum and to producethe second partial layer from copper using direct current deposition. Inthis manner, the deposition procedure and the material costs can bereduced. The thickness of the substrate layer and the cover layer isideally, in each case, less than 10 μm, e.g. approx. 5 μm to 10 μm.

A sensor element can be constructed from individual nanowire structuralelements. The sensor element is suited, or intended, for use, forexample, in measuring gas flow or temperature. Furthermore, the sensorelement may be used as a movement sensor. The sensor element contains:at least one measuring device, which has a first nanowire structuralelement and a second nanowire structural element, wherein in each case,the nanowire elements have a first substrate layer and a secondsubstrate layer for making contact with the respective nanowirestructural elements. A “heat-able/warm-able element” is located betweenthe nanowire structural elements. The heating results, for example, fromapplication of voltage to the element, which, for example, may be aheat-able microwire. For this it is preferable that each nanowirestructural element be connected individually. If a gas is fed throughthe sensor element, said gas is heated by the heated element, and inturn heats the nanowire structural element located behind the element,wherein a change in the resistance of the sensor element results or isinduced. In this manner, the change in resistance is a measure of thegas flow through the sensor element. The change in resistance can alsobe a measure for the temperature change resulting from the gas flow. Ifthe sensor element is moved, the change in resistance indicates a changein position. For this, the gas first passes through the first nanowirestructural element, then the heat-able element and then the secondnanowire structural element. The sensor element may be, for example,produced using a mask during the irradiation of the template used, inother words, using a process in accordance with claim 5.

In the following, the invention will be explained in detail using theembodiment examples and in reference to the illustrations, whereinidentical and similar elements have the same reference symbols in partand the characteristics of different embodiments, particularly theprocedures with and without radiation masks, can be combined with eachother.

SHORT DESCRIPTION OF THE ILLUSTRATIONS

They show:

FIG. 1 An overview of the production of a nanowire structural elementwith a nanowire network.

FIG. 2 A three-dimensional presentation of the deposition device usedfor electrochemical deposition.

FIG. 3 A three-dimensional transparent exploded view of the depositiondevice for the deposition of the cathode layer.

FIG. 4 A three-dimensional transparent exploded view of the depositiondevice for the deposition of the nanowires and, where applicable, acover layer.

FIG. 5 An SEM image of a nanowire structural element with a nanowirenetwork.

FIG. 6 An SEM image of the nanowire structural element from FIG. 5,significantly enlarged.

FIG. 7 A schematic overview of the production of a nanowire structuralelement with a three-dimensional (3-D) nanowire network.

FIG. 8 A TEM image of a nanowire structural element open at two sidesand closed at two sides with a nanowire array of platinum nanowires.

FIG. 9 An SEM image of the three-dimensional nanowire network from FIG.8 slightly enlarged.

FIG. 10 An SEM image of the three-dimensional nanowire network fromFIGS. 8 and 9 enlarged to a lesser degree.

FIG. 11A schematic exploded view of a microreactor with the nanowirestructural element for use in flow operations.

FIG. 12 A schematic presentation of a sensor element with two nanowirestructural elements.

DETAILED DESCRIPTION OF THE INVENTION

Overview of the Production Process

The production of nanowire structural elements is based on a templatebased process. The partial steps of the process are schematicallypresented in FIG. 1 as follows:

-   (a) Preparation of the template foil,-   (b) Irradiation with ions,-   (b1) Application of a gold layer,-   (b2) Electrochemical reinforcement of the gold layer (optional),-   (c) Etching of the ion tracks to form nanopores,-   (d) Deposition of the nanowires in the nanopores,-   (d1) Removal of the cathode layer (optional),-   (e) Dissolving and removal of the template foil.

Ideally, the process steps are carried out in the order shown in FIG. 1,i.e. (a), (b), (b1), (b2), (c), (d), (d1), (e). It is, however,basically possible to use a different sequence, such as, to etch fromtwo sides and subsequently to then first to apply the cathode layerpartial step ((c) before (b1) and (b2)) (see, e.g., FIG. 7).

In accordance with FIG. 1 (b), first a template foil 12 is bombardedwith ions 14, wherein latent ion tracks 16 are generated in thesubstance of the template foil 12 along the trajectory (b1). Thetemplate foil 12 is a polymer foil in this example, specifically, apolycarbonate foil. A special feature of the process described hereconsists of the fact that the template foil is irradiated with ions fromtwo angles in this example. In this example, the template foil isirradiated once at an angle of +45° to the surface of the template foil,and once at −45°, such that the latent tracks and later the intersectingnanopores, or respectively, intersecting nanowires run at an angle of90° to each other. It is to be understood that other angles may also beused.

For successive irradiation of the template foil 12 at different angles,the template foil 12 is first positioned in an appropriate irradiationtube at a first angle to the direction of the ion beam, for example inthe synchrotron of the GSI, and irradiated with a predetermined firstion surface density. Subsequently the template foil 12 is tilted inrelation to the beam direction and irradiated with a secondpredetermined ion surface density. Should it be the case that nanowiresare to be generated at different angles, the process is repeated untilthe angles have been obtained. The density of the ions, or respectively,the surface density necessary for a specific surface density ofnanowires is calculated in advance and determined. The irradiation isthen carried out with this predetermined ion surface density. To producea 3-D network as explained below, a template foil 12 positioned at apolar angle to the beam axis is rotated on the beam axis to the azimuthangle.

Subsequently, on the first side 12 a of the template foil 12, a thin,conductive metallic layer 22 a, e.g. gold, is sputtered onto said (b1),forming a first partial layer. Subsequently, the first partial layer 22a is reinforced electrochemically with a second partial layer 24 a thusforming the cathode layer 26 a (b2), which later serves as a cathode fornanowire deposition (d). For the electrochemical deposition of thesecond partial layer 24 a, the template foil 12 is mounted in thedeposition device 82 shown in FIGS. 2-4.

Subsequently, the template foil 12 coated on one side is then removedfrom the deposition device 82, and the latent ion tracks 16 arechemically etched, wherein the continuous intersecting channels arecreated. These channels are referred to as nanopores and due to theirdifferent directions in the template foil 12, form intersecting andinterconnected nanopores 32. Alternatively, the etching process may alsobe carried out in the deposition device 82, in that the etching solutionis placed in the appropriate cell 88, and the repeated insertions arenot necessary. The diameter of the nanopores 32 can be controlled bycontrolling the etching time period (c).

Following this, the template foil 12 prepared in this manner, permeatedwith the intersecting network 33 of nanopores 32, is placed again in thedeposition device 82, and in a second electrochemical process, thedesired metal for the formation of a network 37 of nanowires 34 in thenanopores 32 is electrochemically depositioned (d). In this example,platinum is depositioned in the nanopores, such that the nanowires 34,or respectively, the nanowire network 37, consist(s) of platinum. Thisplatinum nanowire network 37 forms a catalytic active network due to thecatalytic active surface of the platinum nanowires 34.

After the deposition of the nanowires, or respectively, the generationof the nanowire network 37 in the template foil 12, the cathode layer 26a may be removed if desired (d1). The removal of the cathode layer 26 ais ideally carried out before the template foil is dissolved andremoved. The gold cathode layer used in this example can be readilyremoved from the platinum nanowires. The cathode layer 26 a can,however, remain on the template foil 12, and after the dissolving andremoval of the template foil, forms a substrate layer 27, on which thenanowire network 37 is positioned and firmly joined (see FIG. 7 in thefollowing).

Finally, the polymer foil 12 is dissolved in an organic solvent suitedto this purpose (e). The nanowire structural element 1 produced herebyin accordance with the invention is shown in FIG. 1 (e).

The nanowire structural element 1 contains or consists of a nanowirearray 35 of intersecting, interconnected nanowires 34, which form anintegrally meshed nanowire network 37. The network 37 displays a certainstability due to the meshed structure of the merged together nanowires,even without cover layers, or, in other words, open on all sides, evenwhen cover layers of this typed, e.g. on one side (substrate layer 27)or on both sides to form a sandwich structured are not excluded as apossibility.

The template based method has the advantage that many of the parameterscan be specifically manipulated. The length of the nanowires 34 isdetermined by the thickness of the template 12 used and ideally is10-200 μm, particularly preferred is circa 50 μm+50%. The surfacedensity of the nanowires 34 is determined by the irradiation. Theminimal surface density of the ion irradiation, and thereby thenanopores, should be selected such that a sufficient portion of thenanowires 34 can merge together. In this regard, a preferred surfacedensity for the production of the array is ideally between approx. 1×10⁷cm² and 1×10⁹ cm². The diameter D of the nanowires 34 is determined bythe time period of the etching and may be from ca. 20 nm to 2000 nm. Theaspect ratio may have values of up to 1000.

Possible materials for the nanowires are electroconductive materials,particularly metals or metallic compounds which are suited toelectrochemical deposition. Experience has been made with the followingmetals which have been proven suitable: Cu, Au, Bi, Pt, Ag, Cu, Cu/Comultilayer, Bi₂Te₃.

On the one hand a large number of nanowires 34 with small diameters D isdesired, in order to obtain a large active surface area, and on theother hand a good mechanical stability should be obtained. Theoptimization of this depends on the material used and is adjusted to theneeds accordingly.

For nanowire structural elements 1 with platinum nanowires 34, a stableconstruction is produced with 10⁸ wires per cm² having a diameter of 250nm and a length of 30 μm. The aspect ratio here is 120. Such nanowirestructural elements are suited, for example, for use as catalyticelements due to the catalytic characteristics of for example platinum ofthe other elements contained therein.

Example 1

For the production of a nanowire structural element 1, a 30 μm thickcircular shaped (r=1.5 cm) polycarbonate foil 12 (Macrofol®) irradiatedwith heavy ions 14 having an energy of 11.1 MeV/u and at two angles(+45°, −45° each having a fluence of 5×10⁸ ions/cm² is used. Prior tothe application of the conductive metallic layer 22 a, each side of thepolymer foil 12 is irradiated for one hour with UV light, in order toincrease the selectivity of the etching along the tracks 16.

A gold layer 22 a is sputtered onto the first side 12 a of the polymerfoil 12, having a thickness of ca. 30 nm (b1). This is reinforced by apotentiostatic deposition of copper from a CuSO₄ based electrolytesolution (Cupatierbad, Riedel) with a voltage of U=−500 mV, wherein acopper rod electrode serves as the anode (partial step 24 a) (b2). Thedeposition is stopped after 30 minutes, at which point the copper layer24 a is approx. 10 μm thick. Subsequently, etching is carried out fromthe untreated side 12 b of the template foil 12 at 60° C. with an NaOHsolution (6 M) for 25 minutes and thoroughly rinsed with deionizedwater, to remove residual etching solution. At this point, thenanoporous template foil 12 is mounted in the deposition device 82.

The deposition of nanowires 34 is carried out at 65° C. with alkaline Ptelectrolytes (Pt—OH bath, Metakem). To generate the nanowires 34, theprocess of the reversed pulse deposition is used in order to compensatefor the slow diffusion driven transportation in the nanopores 32, and toobtain uniform development of nanowires 34. Following a deposition pulseof U=−1.3 V for 4 seconds, there is an anodic pulse for 1 second atU=+0.4 V. After a few tens of minutes, the deposition is stopped, andthe development is checked. At this point, the nanowires 34 havedeveloped sufficiently to merge together in the nanopores.

Finally, the template foil is removed, wherein the entire nanowirestructural element 1 with the template foil 12 is placed in a containerwith 10 ml dichloromethane for several hours. In this example, thecathode layer remains as a substrate 27 on the nanowire array 35 andforms a component of the nanowire structural element 1. The solvent isreplaced three times in order to fully remove residual polymers from theinterior 38 of the nanowire array 35.

A nanowire structural element 1 produced in this manner may be seen inthe scanning electron microscope images (SEM) in FIGS. 5 and 6. Thenanowires 34 here have a diameter of approx. 150 nm. Because theirradiation is carried out at two angles it is referred to as2-dimensional. Such a 2-D nanowire network structure may be seentherefore in FIGS. 5 and 6, which has been produced in a template thathas been irradiated twice at different angles (+45°, −45°). It hasformed a network which is relatively stable after removal of the polymermatrix, distributed on the substrate and joined to said.

The enlarged SEM image of a few nanowires 34 in FIG. 6 shows that thenanowires 34 have merged nicely at the nodes 39 and are thereby firmlyjoined and remain in place due to the predetermined radiationorientation of 90°. The nodes 39 where the nanowires 34 have mergedtogether are predetermined by the intersections of the nanopores and aredistributed at one or more places over the length of the nanowires 34,or between the ends of the nanowires 34.

Example 2

In reference to FIGS. 7-10, a further embodiment is produced. FIG. 7shows schematically, and partially summarized the following partialsteps of the process:

(a) Preparation of the template foil,

(b), (c) Irradiation and etching of the ion tracks to form nanopores,

(b1), (b2), (d) Generation of a cathode layer and deposition of thenanowires in the nanopores,

(e) Dissolving and removal of the template foil.

With reference to FIG. 7, the template foil or polymer membrane 12 isirradiated from more than two different directions. The irradiation iscarried out in this example from four different directions, wherein thefour irradiation directions are not in the same plane. With reference toperspective view shown in FIG. 7, radiation is applied in each case oncefrom each of the four sides diagonally from above, and this being at apolar angle of 45° to the surface of the substrate and at azimuth anglesof 0°, 90° 180° and 270°. The template foil 12 is therefore rotated atleast three times during the irradiation.

When the template foil has been irradiated from at least threedirections (in this example, four directions) which are not in a commonplane, a three-dimensional nanopore network 33 (c) and subsequently athree-dimensional nanowire network 37 (d) and (e) can be produced. Inother words, the irradiation directions and thereby the nanowires 34 liein numerous (non-parallel) planes, which are at an angle to the templatesurfaces 12 a, 12 b. With a three-dimensional nanowire network 37generated in this manner, the nanowires accordingly run in threenon-parallel planes, thus forming a three-dimensional interconnectednetwork structure. Expressed mathematically, the nanowires run along atleast three non-parallel axes, which form at least two planes which inturn are not parallel. In this manner, the nanowires are not onlyconnected to other nanowires in the same plane, but nanowires also existwhich run at an angle to one of the planes formed by these nanowires. Inother words, in the language of vector mathematics, it is possible withthe at least three axes, or respectively, nanowire directions, to span athree-dimensional vector space. The, at least three, axes, orrespectively, nanowire directions, are independent on a linear level. Athree-dimensional interconnected nanowire network of this type isreferred to here as a three-dimensional (3-D) nanowire network.

FIG. 8 shows a transmission electron microscope image (TEM) and FIG. 9shows an SEM image with slight enlargement of a three-dimensionalnanowire network 37 of this type, in which it may be seen that amechanically stable, cohesive system of nanowires running at predefinedangles has been formed, which remains erect to a large degree as aresult of the predetermined orientation of the polymer matrix after thetemplate has been removed, which can be seen particularly well in theSEM image in FIG. 9. The template 12 of the network 37 displayed herehas been produced with 4 irradiations from four different linearlyindependent directions in each case with 5×10⁸ ions/cm². The thicknessof the nanowires here is approx. 50 nm.

Aggregation and loss of the active surface area is rarely observed,which means that excellent accessibility and ready catalyzer returns areobtained. The production process allows for a simple controlling of thenetwork parameters by means of adjustment of the diameter of the wires,the integration density and the complexity (number of different nanowiredirections) of the network. It is possible to produce very largenanowire networks 37 which are of several millimeters in two dimensions,as can be seen for example in FIG. 10. FIG. 10 shows an SEM image of theapprox. 1 cm² nanowire structural element from FIG. 9. The entirenanowire structural element 1 is accordingly large on a macroscopiclevel. At the left edge of the image another somewhat smaller nanowirestructural element may be seen. The Pt nanowire networks have a largecatalytic active surface area, without a carrier substrate 27 beingnecessary, although this possibility is not excluded.

The nanowire structural element 1 in accordance with the inventionaccordingly has nanowires 34 connected to a network 37 wherein thenetwork structure, in particular the 3-D network structures display thepossibility for connecting, in a mechanically stable manner, micro- andeven macroscopic structures. The stability is so great that they aresuited for integration without cover layers on both sides, and as thecase may be, even without a carrier substrate. Nearly all nanowires 34are not only mechanically firmly connected, but also connected to eachother in an electroconductive manner, wherein these structures have alarge potential for use in electrocatalysis.

A Further Example Regarding the Deposition Parameters

In another example, the etching period is set at 18 minutes, resultingin nanowires 34 with a diameter of approx. 250 nm. The surface density(number for each surface) here is 10⁸ cm⁻². For the electrochemicaldeposition of the wires, the reversed pulse deposition is used again. Adeposition pulse of U₁=−1.4 V for 40 ms is followed by a shortcounter-pulse of U₁=−0.1 V for 2 ms and a pulse interval of 100 ms at avoltage of U=−0.4 V, corresponding to a surplus voltage of approx. 0 V.This means that during the counter-pulse, the system is in a state ofequilibrium.

Construction for the Electrochemical Deposition

With reference to the FIGS. 2-4 the electrochemical deposition of thenanowire array 35 consisting of numerous nanowires 34 is carried outusing the deposition device 82, which shown in FIG. 2, in its entirety.It consists of a metal housing 84, in which the metal sled containingone of the two electrolysis cells 86, 88 can be inserted. Due to thegood heat transfer properties of metal, it is possible to temper thedeposition device by controlled external heating.

The electrolysis cells 86, 88 made of PCTFE have on their two facingsides, in each case, circular openings 87, 89 of the same size and canbe pressed together firmly with a hand turned screw. A copper ring 92between the two electrolysis cells 86, 88 serves as a cathode, orrespectively, to establish contact with the first cover layer for theelectrochemical deposition.

With reference to FIG. 3, for electrochemical reinforcement of thepartial layer 22 a, the ion track etched template foil 12 is mountedbetween the two electrolysis cells 86, 88 such that the partial layer 22a, in this case, the sputtered gold layer 22 a, establishes good contactwith the ring shaped copper electrode 92. On both sides of the copperring used as a cathode, electrolytes are injected into the electrolysiscells. The electrochemical reinforcement of the gold layer 22 a on thefirst cover layer 26 a is carried out with a first anode 94, which isplaced in the electrolysis cell 86 facing the partial layer 22 a, and anexternal power source with a control device.

After removing the template foil 12 and etching the nanopores 32 outsideof the deposition device 82, the template foil 12 is placed again in thedeposition device 82.

With reference to FIG. 4, the template foil 12 which has been coated onone side and made porous is again placed in the deposition device 82 asin FIG. 3 for electrochemical deposition of the nanowires 34 and, whereapplicable, the completion of cover layer opposite the cathode layer 26a, such that the cover layer 26 a makes contact with the ring electrode92. At this point, deposition is carried out on the second side 12 b ofthe template foil 12 with a second anode 96 located in the electrolysiscell 88 on the side away from the cathode layer 26 a.

Examination of the Influence of the Electrochemical DepositionConditions to the Development of the Nanowires

With the pulsed deposition procedure for generating nanowires 34, auniform length of the nanowires can be advantageously obtained at anypoint in time of the deposition. This can be explained, without claim tocompleteness and accuracy, in that the diffusion layers are keptrelatively short in comparison to direct current deposition. In theintervals (equilibrium or counter-pulse) between the deposition pulses,metal ions in the nanopores 32 can re-diffuse such that on the entireelectrode surface a nearly uniform concentration is obtained at thebeginning of each deposition pulse, which results in a homogenousdevelopment. The diffusion layers barely overlap each other andirregularities in the surface are not enhanced.

Structural Characteristics of the Nanowires

In the framework of the invention the structural characteristics of thenanowires 34 made of different materials are also studied. Withelectrochemically depositioned material it is possible, for example, tocontrol the size of the crystallite. This affects the mechanicalstability, the thermal and electrical transference characteristics aswell as the surface area and thereby also the catalytic activity. Manycharacteristics can thereby be strategically influenced.

In particular, the structure of the nanowires 34 is studied using X-raydiffraction. For this, the texture as a function of the electrochemicaldeposition is analyzed.

Pt nanowires 34 produced using direct current show a clear <100>texture. The texture coefficient TC₁₀₀ is 2.32, wherein the maximumvalue is 3. The size of the crystallite is determined by the half-widthof the platinum signal by means of the Scherrer equation, and is 8 nm.For catalytic application, the smallest possible crystallite is desired.The value given here lies in the range of the nanoparticles otherwiseused for catalysis. Based on this it may be assumed that the crystallitesize can be reduced even more through modification of theelectrochemical deposition conditions.

When studying nanowires 34 which are produced using pulsed deposition,one finds no specific texture. The intensity of the signals correspondsto those of polycrystalline platinum.

Finally, a sample produced using reversed pulse deposition, is studied.This also shows a clear <100> texture, wherein the texture coefficientTC₁₀₀ is 4.6. The crystallites display accordingly a preferredorientation, wherein the degree of the alignment is 83%. An alignment ofat least 50% in this case is advantageous.

The characterization by means of X-ray diffraction of nanowires 34produced using different means has shown that the deposition conditionshave an effect on the texture. Therefore, the structure of the nanowirecan be strategically influenced.

The surface of a nanowire 34 does not correspond to smooth surface of acylinder, which is the basis for the calculation of the geometricalsurface, but rather, it displays numerous recesses and swellings in itscontour which significantly increases the surface area. The actual sizeof the surface area is therefore typically larger than the geometricalsurface area, because, among other reasons, the crystallites from whichthe nanowires 34 are constructed are very small. In order to obtain amore precise idea of the surface area of the nanowire arrays 35,cyclovoltammetric measurements at 60° C. in 0.5 M H₂SO₄ are carried outfor a potential range of 0-1,300 mV with a standard hydrogen electrode.From the load in which the adsorption of hydrogen is transmitted, it ispossible, taking into account the capacitive currents, to calculate thesurface area of the electrodes. The cyclovoltammetric examination ofnanowire arrays shows that the actual surface area is greater than thegeometrical surface area by a factor ranging from 4-5.

Applications

As a catalyzer it is possible to connect a series of numerous nanowirestructural elements 1 according to the invention. Based on measurements,the nanowire structural element 1 is suited individually for applicationin microstructured systems having three-dimensional structures whereinthe internal measurement is less than 1 mm and for the most part liesbetween ten and a few hundred micrometers.

FIG. 10 shows a schematic illustration of a microcatalyzer 100, in whicha nanowire structural element 1 according to the invention is placedbetween a fluid intake 102 and a fluid discharge 104. It is conceivablethat in a microcatalyzer 100 of this sort gas or fluid phase reactionscan be carried out. For this purpose, a gas or fluid flow is directedunder pressure through the microcatalyzer 100.

The nanowire structural element 1 produced according to the inventionfurthermore inherently contains an electric contact to all of thenanowires. As a result, a controlled voltage may be applied to thenanowires 34 thereby enabling Electrocatalytic processes. Furthermore,the component may be used as an amperometric sensor.

Production of Microelements using a Radiation Mask

In accordance with the invention, it is possible to create nanowirestructural elements or nanowire arrays of very small sizes, in that thetemplate foil 12, a polymer foil in this example, is irradiated withheavy ions through a corresponding mask. The mask, e.g. a perforatedmask, which is already applied contains numerous openings orperforations, wherein each opening defines a future microelement 1 a.The mask covers the template foil 12 during the irradiation, and latention tracks 16 are formed thereby, which are subsequently etched to formnanopores 32 only in the areas which are not covered by the mask, i.e.at the openings of the mask. The layout and the shape of themicroelement 1 a are determined therefore by the mask.

This process is specifically for the production of many very smallnanowire structural elements, as stated, in the form of microelements.The microelements which may be produced in this manner consist of a 2-or 3-dimensional nanowire network 37 which may have a size of less than500 μm, and particularly less than 100 μm, and where applicable, evenless, to a size of only a few micrometers.

For example, a perforated mask for the ion irradiation withapproximately 2,000 perforations is placed on the entire depositionsurface of approximately 0.5 cm², such that approximately 2,000microelements with nanowire arrays in islands in the template foil 12can be created at once. After removal of the cathode layer, themicroelements are separated from each other, and when the template foilhas been dissolved and removed, are no longer attached to each other. Itis, however, also possible to implement further steps, e.g. in order togenerate cover layers for each individual microelement.

Because all nanowires 34 have electrical contact at both ends, themicroelements with nanowire arrays are suited for production ofminiaturized sensors. Due to the large number of wires, not only a highsensitivity but also a defect tolerance should result.

Further Applications

In particular, the microelements are suited for the production of sensorelements, e.g. for measuring gas flow, temperature and for use asmovement sensors. In reference to FIG. 12, such a sensor 150 has atleast one measuring unit with a first and second nanowire structuralelement 1 a, wherein the nanowire structural element 1 a in each casehas a cover layer 27 on each side, wherein each of the two nanowirestructural elements 1 a establishes electrical contact by means of oneor both of the cover layers 27, wherein the two nanowire structuralelements 1 a establish electrical contact separately. A heating elementis located between the two nanowire structural elements, e.g. amicrowire 152 which can be heated through application of voltage.Modification of the resistance of the sensor element 150 is used as ameasure for the gas flow, the temperature or the change in movement.

It is clear to the person skilled in the art that the precedingdescriptions of embodiments are to be understood as exemplary, and thatthe invention is not limited to said, but rather, can be varied innumerous ways, without abandoning the scope of the invention. Inparticular, the production of a microcatalyzer is only one of many usesfor the nanowire structural element of the invention. Furthermore, it isclear that the characteristics are, regardless of whether they arepresented in the description, the claims, the illustrations orotherwise, also define significant components of the invention, even ifthey are described in conjunction with other characteristics.

The invention claimed is:
 1. A nanowire structural element that includesan array of numerous nanowires running in different directions andwherein the nanowires intersecting at numerous nodes have merged suchthat the nanowires are interconnected to form a network wherein thenanowires run along at least three predefined different directionswithin the network at 90° to each other, and wherein the at least threepredefined different directions do not lie in a common plane, formingwith the at least three predefined different directions a vector space,and the nanowires are interconnected in a three-dimensional manner andwherein the element is freestanding.
 2. A nanowire structural elementaccording claim 1, wherein the nanowire network is generated fromelectrochemically depositioned material.
 3. A nanowire structuralelement according to claim 1, characterized in that it has a flat,mat-like shape.
 4. A nanowire structural element according to claim 3,which does not have a cover layer on at least one side, and wherein thenanowire network is open on at least this one side.
 5. A nanowirestructural element according to claim 1 which furthermore contains asubstrate layer with which the nanowire network is firmly joined.
 6. Ananowire structural element according to claim 1, wherein the nanowireshave a crystallite texture or a uniform crystalline structure.
 7. Ananowire structural element according to claim 1, wherein the predefineddifferent directions are irradiation-defined.
 8. A nanowire structuralelement according to claim 1, wherein the nodes are predetermined.
 9. Ananowire structural element according to claim 1, wherein the elementhas a stable hollow chamber-like structure.
 10. A nanowire structuralelement that includes an array of numerous nanowires running indifferent directions and wherein the nanowires intersecting at numerousnodes have merged such that the nanowires are interconnected to form anetwork wherein all of the nanowires run along at least three predefineddifferent directions within the network and wherein the at least threepredefined different directions do not lie in a common plane, formingwith the at least three predefined different directions a vector space,and the nanowires are interconnected in a three-dimensional manner andwherein the element is freestanding, wherein the at least threepredefined directions at predefined angles includes a polar angle of 45degrees and azimuth angles of 0 degrees, 120 degrees and 240 degrees.11. A nanowire structural element according to claim 10, wherein theintersecting nanowires run at an angle of 90 degrees to each other. 12.A nanowire structural element according to claim 10, wherein thenanowires are greater than 50 nm in diameter.